1----A solution contains 7.65×10 -3 M ammonium carbonate and 1.21×10 -2 M potass

Question

1----A solution contains 7.65×10-3 M ammonium carbonate and 1.21×10-2 M potassium hydroxide.
Solid copper(II) nitrate is added slowly to this mixture.

What is the concentration of hydroxide ion when carbonate ion begins to precipitate?
[hydroxide] =

2----A solution contains 1.49×10-2 M nickel(II) acetate and 7.55×10-3 M zinc nitrate.
Solid potassium cyanide is added slowly to this mixture.

What is the concentration of nickel ion when zinc ion begins to precipitate?
[Ni2+] =

3----A solution contains 5.40×10-3 M potassium chloride and 1.13×10-2 M sodium cyanide.
Solid silver nitrate is added slowly to this mixture.

A. What is the formula of the substance that precipitates first?

B. What is the concentration of silver ion when this precipitation first begins?
[Ag+] =

4----A solution contains 1.06×10-2 M silver acetate and 1.05×10-2 M nickel(II) nitrate.
Solid sodium cyanide is added slowly to this mixture.

A. What is the formula of the substance that precipitates first?

B. What is the concentration of cyanide ion when this precipitation first begins?
[CN-] =

Explanation / Answer

Q.1: When Copper(II) nitrate is added, it forms the precipitation of CuCO3 and Cu(OH)2.

The Ksp of CuCO3 is 2.5x10-10   

The Ksp of Cu(OH)2 is 1.6x10-19  

Since th esolubility of Cu(OH)2 is very low it will precipitate first and then CuCO3 will be precipitated,

For the dissociation of CuCO3

---------- CuCO3(s) + H2O ------ > Cu2+(aq) + CO32-(aq); Ksp = 2.5x10-10

Eqm.conc: ----------------------------- S ------------ S

Ksp = 2.5x10-10 = [Cu2+(aq)]x[CO32-(aq)] = S2

=> S = squareroot(2.5x10-10) = 1.58x10-5 M

Hence the concentration of Cu2+(aq) when the carbonate ion begin to precipitate is 1.58x10-5 M.

Now for the dissociation of Cu(OH)2,

---------- Cu(OH)2(s) + H2O ------ > Cu2+(aq) + 2OH-(aq); Ksp = 1.6x10-19  

Eqm.conc: -------------------------------1.58x10-5 M,

Ksp = 1.6x10-19 = [Cu2+(aq)]x[OH-(aq)]2 = 1.58x10-5 x [OH-(aq)]2

=> [OH-(aq)] = 1.01 x 10-7 M (answer)

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